1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell, and more particularly, to a stable, reliable, rechargeable non-aqueous electrolyte secondary cell having a high energy density and an extended charge/discharge life.
2. Discussion of Prior Art
A number of proposals have been made on high energy density cells using lithium as negative electrode active material. There are already commercially available lithium cells which use graphite fluoride or manganese dioxide as the positive electrode active material. These cells, however, are primary cells and cannot be charged again.
Secondary cells using lithium as negative electrode active material are also known. They use chalcogenides (sulfides, selenides or tellurides) of titanium, molybdenum, niobium, vanadium, and zirconium as the positive electrode active material. However, few cells were commercially available because cell performance and economy are unsatisfactory. Recently, a secondary cell using molybdenum sulfide has been commercially marketed although it has a low discharge potential and is liable to failure by over-charging. The positive electrode active materials which are known to provide a high discharge potential are chromium oxide and vanadium pentoxide. However, chromium oxide has poor charge/discharge cycle performance and vanadium pentoxide is less conductive and hence, unsatisfactory in cathode properties.
Japanese Patent Application Kokai No. 50-54836 discloses a non-aqueous electrolyte secondary cell comprising a negative electrode of metallic lithium and a positive electrode of titanium disulfide TiS.sub.2. This cell, however, has a potential as low as 2 volts on average because the redox potential of titanium disulfide is low. The cell also has poor cycle performance because dendrites generate on the metallic lithium negative electrode upon charging.
To control the dendrite formation, Japanese Patent Application Kokai No. 52-5423 proposes to use a lithium-aluminum alloy containing 63 to 92 atom% of lithium as the negative electrode. The resulting cell is improved in cycle performance, but has a further lowered potential.
On the other hand, a secondary cell having a high potential is obtained by using metallic lithium as the negative electrode and vanadium pentoxide V.sub.2 O.sub.5 as the positive electrode as disclosed in Japanese Patent Application Kokai No. 48-60240 and W. B. Ehner and W. C. Merz, Roc 28th Power Sources Sympo., June 1978, page 214. This cell, however, is also poor in cycle performance because the negative electrode is formed of metallic lithium. Another problem is that vanadium pentoxide is less conductive. When metallic lithium for the negative electrode is replaced by a lithium-aluminum alloy, the resulting cell is substantially improved in cycle performance, but undesirably has a reduced capacity at a voltage of 2 volts or higher.
One primary use of lithium secondary cells is a backup power supply for IC memories. In such application, a voltage of less than 2 volts is insufficient for a cell to function as a memory backup. The capacity of a cell at a voltage of 2 volts or higher is important.
There is a need for a stable, reliable lithium secondary cell having a high energy density and an extended charge/discharge life.